Technical Field
The present invention relates to a semiconductor device and a method of manufacturing a semiconductor device.
Background Art
Silicon (Si) is conventionally used as the constituting material for power semiconductor devices, which control high voltages and large currents. There are several types of power semiconductor devices, such as bipolar or IGBTs (insulated gate bipolar transistors) and MOSFETs (metal oxide semiconductor field effect transistors), each specialized for different uses.
For example, bipolar transistors or IGBTs have higher current density than MOSFETs and can have large currents, but cannot be switched at high speeds. Specifically, bipolar transistors are limited to switching frequencies of several kHz, and IGBTs are limited to switching frequencies of several dozen kHz. On the other hand, power MOSFETs have lower current density than bipolar transistors or IGBTs and cannot easily have large currents, but can have high-speed switching operations of up to several MHz.
However, there is strong demand in the market for a power semiconductor device that has both large currents and high speed, and thus effort has been poured into improving IGBTs and power MOSFETs, to the point now where development has almost reached material limits. Therefore, there has been research into semiconductor materials to replace silicon in power semiconductor devices, and silicon carbide (SiC) has garnered attention as a semiconductor material that makes possible the fabrication (manufacturing) of next-generation power semiconductor devices with excellent low ON voltage, high speed properties, and high-temperature properties (see Non-patent Document 1 below, for example).
Silicon carbide is a very chemically stable semiconductor material, with a wide bandgap of 3 eV and capable of extremely stable use as a semiconductor even at high temperatures. Furthermore, silicon carbide also has a maximum electric field strength that is at least an order of magnitude greater than silicon, and thus shows promise as a semiconductor material that can sufficiently minimize ON resistance. These advantages of silicon carbide also apply to other semiconductors with wider bandgaps than silicon (hereinafter, “wide bandgap semiconductors”), such as gallium nitride (GaN). Hence, the use of wide bandgap semiconductors makes it possible to increase the breakdown voltage of semiconductor devices (see Non-patent Document 2, for example).
In this type of high breakdown voltage semiconductor device, a high voltage is applied not only to the active region where the device structure is formed and current flows in an ON state, but also to an edge termination area for maintaining the breakdown voltage disposed on the periphery around the active region, with the electric field being concentrated at the edge termination area. The breakdown voltage of a high breakdown voltage semiconductor device is determined by the impurity concentration, thickness, and electric field strength of the semiconductor, and the area of safe operation determined by the advantages inherent in semiconductors is the same from the active region to the edge termination area. Thus, there is a risk that the electric field concentrating at the edge termination area could cause an electrical charge exceeding the area of safe operation to be applied to the edge termination area, resulting in destruction of the structure. In other words, the breakdown voltage of the high breakdown voltage semiconductor device will be limited at the area of safe operation of the edge termination area.
There are well-known devices that reduce or disperse the electric field of the edge termination area in order to improve the breakdown voltage of the entire high breakdown voltage semiconductor device, such as devices having edge termination structures such as junction termination extension (JTE) structures or field limiting ring (FLR) structures in the edge termination area (see Patent Documents 1 and 2 below, for example). There is also a well-known semiconductor device that has floating metal electrodes in contact with the FLRs arranged as field plates (FPs) so as to discharge charge that is generated at the edge termination area, thereby improving reliability of the device (see Patent Document 1 below, for example).
However, a semiconductor device constituted by a wide bandgap semiconductor has a device structure that is formed on a semiconductor substrate with a higher impurity concentration than a semiconductor device made of silicon (see Non-patent Document 2 below, for example). Therefore, when forming the edge termination area with the FLR structure, the design requires microstructures of 1 μm or below, depending on the parameters, which makes it difficult to use a wide bandgap semiconductor due to the trouble of forming such microstructures therewith.
There are well-known methods to solve these problems, such as by using a structure in which areas of differing concentrations are nested therein (see Patent Documents 3 and 4 below, for example) or a structure having a high concentration p-type region arranged in a ring shape in a low concentration p-type region (see Non-patent document 3 below, for example).